With seemingly few clues and an "enormous" search zone from the border of Kazakhstan and Turkmenistan in Central Asia to southern Indian Ocean, "all efforts are now to reduce the area of concentration", Acting Transport Minister Hishammuddin Hussein said on Tuesday.

This is being done by reviewing available satellite data and asking countries with satellite capabilities for help. A few have responded, he said without naming them.

Malaysia is also asking countries the plane may have flown over to check their radar data.

In the southern corridor, which includes Indonesia and the Indian Ocean, Malaysia is seeking assistance from the US.

"We know the United States has got possibly the best ability to assist us in locating the aircraft in the southern corridor," said Datuk Seri Hishammuddin.

In all, 25 countries are involved in the search for MH370 which disappeared about 50 minutes after it left Kuala Lumpur for Beijing. The Boeing 777- 200ER was carrying 239 people.

On Tuesday, Mr Hishammuddin also clarified media reports about the plane's Aircraft Communications Addressing and Reporting System (ACARS). This uses radio or satellite signals to send data that monitors engines and other equipment. It also enables aircraft to be tracked while in flight.

He said the precise time the system was disabled "has no bearing" on the search, which is the priority. The important point is that the last satellite transmission was at 8.11am, or 6½ hours after Malaysian air traffic control lost contact.

Aviation and security experts have said it is important to find out when the ACARS was deactivated because the last communication which came from the co-pilot was at 1.19am. If the reporting system was turned off before that, it could suggest that the pilot was responsible for the plane's diversion, they say.

Asked about reports that the route the aircraft took after breaking contact with traffic control was pre-programmed during the flight, Malaysia Airlines chief executive Ahmad Jauhari Yahya said: "Once you are in the aircraft, anything is possible."